Threshold selection circuit

ABSTRACT

A threshold selection circuit, suitable for a processing unit for the processing of video signals obtained from an angle tracking device, is provided with a number (N) of video integrators to receive video signals above certain threshold levels decreasing in numerical order, and with a weighting network, connected to the video integrators, for selecting and assigning the threshold of the lowest numbered video integrator for the processing unit, of which video integrator the output signal does not exceed a fixed fraction of the output signal of the video integrator operating with the next lower threshold.

BACKGROUND OF THE INVENTION

The invention relates to a threshold selection circuit suitable for a processing unit for the processing of video signals obtained from an angle tracking device, such as a television angle tracking device.

Only video signals which exceed a given threshold, derived from a threshold selection circuit, are selected for application to the processing unit; optimal results are thereby obtained if the threshold level is set just above the black level of the video signals supplied.

Such a threshold selection circuit is commonly known and may, for example, consist of a manually adjustable resistance divider circuit. Further it may comprise a manually controlled stepping switch to select the threshold from a number of discrete voltage levels.

A disadvantage of such a selection circuit is that the threshold selection occurs manually, encumbering the acquisition of optimal results. This disadvantage is particularly felt when, through an abrupt jump in the average intensity level of the video signals, a rapid adjustment of the threshold level is required.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a solution to eliminate this disadvantage.

According to the invention, the threshold selection circuit as set forth in the opening paragraph is provided with a number (N) of video integrators, which are fed with video signals and which are active above certain threshold levels decreasing in numerical order, and with a weighting network, connected to the video integrator, for selecting and assigning the threshold of the lowest numbered video integrator for the processing unit, of which video integrator the output signal does not exceed a fixed fraction of the output signal of the video integrator next in order.

DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanying figures, of which:

FIGS. 1A-1D illustrate four television pictures obtained from standardised video signals of one and the same object, but using different thresholds increasing in value;

FIG. 2 illustrates an embodiment of a threshold selection circuit according to the invention;

FIG. 3 illustrates a television picture composed of quantised video signals with contrast gradiation; and

FIG. 4 illustrates a circuit that, utilising the standardised video signals of the threshold selection circuit, generates control signals to obtain the quantised video signals for the picture of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1A-1D show four television pictures obtained from standardised video signals of an aircraft flying in clouds. These pictures are distinguished by using a different threshold for each picture when quantising the video signals. In a preferred embodiment of the threshold selection circuit, a constant difference factor is maintained between the successive thresholds. It should further be noted that in this description a video signal will be regarded as a stronger signal as the signal originates from a darker shaded section in the object field. Because strong video signals cause associated picture line segments to light up, a negative picture is in fact generated. Since black lines are used to display the picture line segments of FIS. 1A-1D on the television screen, it is still achieved that dark-shaded sections of the television screen correspond with dark-shaded sections in the object field. A weak video signal, which originates from a bright part in the object field, is eliminated only if the amplitude of this video signal is below the selected threshold level. For example, in establishing FIG. 1A, where a low threshold level V_(A) is applied, only the weak video signals from the very bright part of the sky above the erratic diagonal line are suppressed. The remaining video signals are fed in standardised form to the television screen. In the picture of FIG. 1A little is noticeable of anything that looks like an aircraft. Even at point P, near the center of the erratic diagonal line, the outlines of something that later will appear to be the tail of the aircraft is hardly distinguishable. Hence, this picture is not suitable to supply any information on dimensions and shape of the aircraft.

In FIG. 1B, where a higher threshold level V_(B) is applied, the video signal strength is above the selected threshold. Besides the bright part indicated in FIG. 1A, the picture of FIG. 1B also lacks some less bright parts of the object field, such as the cloud banks below the erratic diagonal line (see FIG. 1A); only the lower and the right-hand sides of the screen still show two dark cloud formations Q and R. The shape of the aircraft comes into greater prominence where, in addition to the tail P, also the right-hand side wing S and the tail plane T of the aircraft are recognisable. The other side of the aircraft is however obscured by the cloud bank R.

In FIG. 1C, where a still higher threshold level V_(C) is employed, it is seen that the cloud banks Q and R have fully disappeared; however, also the brighter parts of the aircraft, such as the major part of the tail and the rear part of the fuselage are no longer visible on the screen.

In FIG. 1D with threshold level V_(D), only the darkest parts of the aircraft are left; the remaining video signals from the aircraft have been eliminated because of their low strength.

In tracking an aircraft with a television camera it is tried to keep the centroid of the aircraft picture area at the screen center as well as possible. With a clear sky the threshold setting need not be critical; the number of video signals required to obtain a picture on the screen is thereby unaffected. Hence, the centroid of the aircraft picture on the television screen remains unchanged.

With the aircraft flying in clouds, a different situation arises. A correct threshold setting enables one to utilise the largest possible number of video signals from the aircraft with a minimum number of video signals from the clouds in order to determine the centroid of the aircraft picture on the television screen. An exposition of the problems encountered in finding a suitable threshold level, as well as a solution thereto, is given in the following.

The above description already put forward that the threshold level V_(A), which resulted in the picture of FIG. 1A, is less suitable for obtaining information on the object or aircraft on the screen. The threshold levels V_(B), V_(C) and V_(D), which produced the pictures of FIGS. 1B, 1C and 1D, lend themselves much better for this purpose, where threshold V_(C) is slightly preferable to the others.

It is accordingly an object of the present invention to provide a circuit that enables the selection of the best suitable threshold level for optimal processing of the video signals obtained with the television camera. The principles that form the basis of such a circuit will now be described with reference to FIGS. 1A-1D. The television pictures obtained with the threshold levels V_(A) -V_(D) are utilised to establish the area of the picture displayed, the so-called video area. Then, wherever possible, the value of each of the video areas is compared with a fixed fraction of the adjacent video area produced by the next lower theshold level. For the preferred embodiment here depicted, the fixed fraction has a value of 1/2. The highest threshold, pertaining to a video area which does not exceed the fixed fraction of the video area pertaining to the lower threshold, is selected and assigned as the threshold level for a processing unit connected to the above-mentioned circuit.

Suppose the ratio between the video area of the picture in FIG. 1D and the video areas of the pictures in FIGS. 1A-1C is as 32:12:5:3. Hence, the video area of the picture in FIG. 1D is greater than half of the video area of the picture in FIG. 1C. The latter video area is smaller than half of the video area of the picture in FIG. 1B. Therefore, the threshold level V_(C) should be selected and assigned to a processing unit connected to the threshold selection circuit. Such a selection procedure should be performed as frequently as possible.

A threshold selection circuit based on the above principles is illustrated in FIG. 2. This threshold selection circuit comprises a number (N) of video integrators 1.1-1.N, which receive the video signals via inputs 2.1-2.N. Through a second input, each video integrator 1.i, where i=1, . . . , N, also receives a threshold V_(i). These thresholds V₁ -V_(N) may be derived from a voltage divider 3, formed by resistances R₁ -R_(N). The values of resistances R₁ -R_(N) are chosen so that they produce the desired threshold levels V₁ -V_(N).

In the preferred embodiment of the threshold selection circuit, the video integrator, 1.i each comprise a combination of a comparator 4.i and, connected thereto, an integrator circuit 5.i. In the comparator 4.i the applied video signals are compared with the corresponding threshold level V_(i) ; in case a video signal exceeds the threshold level, a so-called standard pulse - standardised in amplitude--is delivered to the integrator circuit 5.i. The delivery of such standard pulses to integrator circuit 5.i results in an output voltage which is a measure for the size of the video area determined by the standard pulses. The output signals of each of the video integrators 1.i are subsequently applied to a weighting network 6, which selects the video integrator 1.i operating with the highest possible threshold level, where the output voltage representing the video area does not exceed a fixed fraction (one-half in this case) of the output voltage of the video integrator 1.i operating with the next lower threshold level.

In the preferred embodiment the weighting network 6 comprises a number of voltage dividers 7.2-7.N, a number of comparators 8.1-8.N-1, and a priority selection circuit 9. The comparator 8.k, where k=1, . . . , N-1, receives both the output voltage of the corresponding video integrator 1.k and the above-mentioned fixed fraction of the output voltage of integrator 1.k+1, as supplied by voltage divider 7.k+1.

In the preferred embodiment of the threshold selection circuit, the two resistances in the voltage divider 7.k+1 are of equal value. The output voltage of the video integrator 1.k and the tapped voltage of the divider 7.k+1 are fed to comparator 8.k, which produces a logically true signal only if the output voltage of video integrator 1.k does not exceed the tapped voltage of divider 7.k+1. Since only N-1 voltage comparisons can be made, the number of comparators 8.k is equal to N-1.

The priority selection circuit 9, connected to comparators 8.1-8.N-1, examines the logical output voltages of these comparators 8.k in succession, starting with the output voltage of comparator 8.1.

A preferred embodiment of such a priority selection circuit 9 is obtained by means of logic memory elements 10.1-10.N-1, which may include flip-flop circuits. Priority selection circuit 9 contains a number of outputs Q₁ -Q_(N-1). The truth table of an output Q_(k) is given by the Boolean logical formula: ##EQU1## where D_(k) and D_(x) represent the output signal of comparators 8.k and 8.x, respectively. Priority selection circuit 9 permits the selection of the video integrators 1.i with the highest possible threshold, of which video integrators 1.i the output voltage does not exceed the fixed fraction of the output voltage of the next higher video integrator 1.i operating with the next lower threshold. The associated threshold level is then selected as well, whereby the video signals are processed during a subsequent period T. Further, a new selection procedure is initiated to obtain a new threshold for the subsequent period T. This period may for instance comprise a complete frame period, i.e. the time duration for the coverage of a single television picture.

The preferred embodiment here depicted needs no additional comparator circuit for the processing of video signals exceeding the selected threshold. Since the comparator of the video integrator, of which the threshold level is also the selected value, already produces the required standardised video signals. An advantageous embodiment of a threshold selection circuit is therefore obtained if this circuit is provided with a number of gate circuits 11.1-11.N-1 and an OR circuit 12 connected to these gate circuits. Gate circuits 11.1-11.N-1 are supplied with the output signals of the corresponding comparators 4.1-4.N-1. By driving each gate circuit 11.k with the corresponding output signal Q_(k) of the priority selection circuit 9, only the standardised video signals from the comparator 4.i of the selected video integrator 1.i will appear at the output of OR gate 12.

Instead of voltage divider 3, which is capable of producing a wide range of thresholds from low to high, it is also possible to use a voltage divider for a limited voltage range, within which the threshold level to be selected may definitely be expected.

To realise such a design of a voltage divider 3, a peak value detector 13 and a mean value detector 14 can be used to advantage. Peak value detector 13 registers the peak value of the video signals to be supplied during a period T and at the end of this period produces a voltage V_(A), representing the peak value, for application to an emitter follower 15. On the other hand, mean value detector 14 registers the mean value of the incoming video signals and, at the end of period T, produces a voltage V_(B), representing the mean value, for application to an emitter follower 16. The outputs of emitter followers 15 and 16 are connected to the voltage divider 3, across which the voltage differences V_(A) -V_(B) is impressed.

It is advantageous to insert a resistance R between emitter follower 16 and resistance R_(N) of voltage divider 3. Resistance R should be greater than each of the resistances R₁ -R_(N) of voltage divider 3. This will prevent, on occurrence of a rapid change in intensity, a greater than usual amount amount of noise signals, clutter signals and other weak video signals from contributing to the video areas determined by the video integrator, since the threshold level of the video integrators 1.i will not vary simultaneously with the intensity change. In such a case, the threshold selection procedure may be impaired, with the result that for example the centroid of the television picture is incorrectly established.

Using a so-called voltage combination circuit 17, it is possible to derive a control signal C with brightness data for the television display from standardised video signals which appear simultaneously at the outputs of the several video integrators 1.i. Such control signals can be utilised to produce a digitised video picture (see FIG. 3). Instead of the circuits of the type that sum or otherwise combine the simultaneously appearing standardised video signals of several video integrators to form a control signal determining the display brightness, there are also circuits of the type that make the amplitude of the output signal dependent upon the standardised video signal obtained with the highest possible threshold level, that is from the video integrator with the lowest possible index.

An example of the latter circuit type is given in FIG. 4. This circuit comprises a first series of diodes 17.1-17.N and a second series of diodes 18.1-18.N, where the anodes of the diodes in the different series but with the same index are interconnected. Further the cathodes of diodes 17.1-17.N are separately connected to bias voltages P₁ -P_(N) respectively, where P₁ >P₂ >. . . >P_(N), while the cathodes of diodes 18.1-18.N are interconnected and biassed, via a resistance R_(u), by a voltage P_(O), where P_(O) >P_(N). The line between the anodes of two identically indexed diodes (17.i, 18.i) is connected to the output of the comparator with a corresponding index (4.i). Thus the common cathode line C of diodes 18.1-18.N will be energised by a voltage derived from the bias voltage of which the index corresponds with that of the highest possible threshold, which still produces a standardised video signal. 

I claim:
 1. Threshold selection circuit suitable for a processing unit for the processing of video signals obtained from an angle tracking device, wherein said threshold selection circuit comprises a number (N) of video integrators which are each fed with the video signals and which are active above respectively differing threshold voltage levels which decrease with the ascending numerical order, 1 to N, of said video integrators, and a weighting network, connected to the video integrators, for selecting the video integrator having the highest threshold voltage level which output signal therefrom does not exceed a fixed fraction of the output signal of the video integrator operating with the next lower threshold voltage level.
 2. Threshold selection circuit as claimed in claim 1, wherein said weighting network comprises a plurality (N-1) of first comparators, each connected directly to the respective first N-1 video integrators and also each being fed with said fixed fraction of the output signal of the respective video integrator having the next lower threshold voltage level, and a priority selection circuit, connected to said first comparators, for selecting the comparator which is directly connected to the video integrator having the highest possible threshold voltage level in which the output signal of the directly connected video integrator does not exceed said fixed fraction of the output signal of the video integrator also aplied to the selected first comparator.
 3. Threshold selection circuit as claimed in claim 2, wherein each of said video integrators comprises a second comparator and an integrator connected thereto, said second comparator being supplied with video signals and being active above the appropriate threshold voltage level.
 4. Processing unit for the processing of video signals obtained from an angle tracking device, which processing unit comprises a threshold selection circuit as claimed in claim 3, wherein said processing unit further comprises N-1 gate circuits, connected separately to the N-1 first comparators and controlled by said weighting circuit, which weighting circuit activates that gate circuit which is connected to the video integrator with the selected and assigned threshold voltage level.
 5. Threshold selection circuit as claimed in claim 1, wherein the threshold selection circuit further comprises a voltage divider, inserted between two potentials, for deriving said thresholds.
 6. Threshold selection circuit as claimed in claim 5, wherein the threshold selection circuit further comprises a peak value detector, supplied with the video signals, for generating the greater of said two potentials.
 7. Threshold selection circuit as claimed in claim 5, wherein the threshold selection circuit further comprises a mean value detector, supplied with the video signals, for generating the smaller of said two potentials.
 8. Threshold selection circuit as claimed in claim 7, wherein said mean value detector functions as an integrating network, and wherein the threshold selection circuit further comprises a gradient control for adapting step-by-step the video signals, to be applied to the mean value detector, to the video gate size.
 9. Threshold selection circuit as claimed in claim 5, wherein said voltage divider comprises a plurality of similar first resistances.
 10. Threshold selection circuit as claimed in claim 9, wherein said voltage divider further comprises a second resistance connected to the smaller of said two potentials, which second resistance is greater than said first resistances.
 11. Video monitor for the processing of video signals obtained from an angle tracking device, which monitor is thereto provided with a threshold selection circuit as claimed in claim 3, wherein the video monitor has a brightness circuit which comprises a voltage combination circuit connected to the second comparators of the video integrators. 